Additive for silver-palladium alloy electrolytes

ABSTRACT

The present invention relates to an electrolyte containing suitable reducing agents for adjusting the composition of silver-palladium layers. Furthermore, these reducing agents contribute to improving the layer appearance and to increasing the luminance (L value, CIE Lab) of the deposited layers. The present invention also discloses a method for the electrolytic deposition of silver-rich silver-palladium alloys. The alloys can be deposited on conductive surfaces over a wide current density range.

The present invention relates to an electrolyte containing suitablereducing agents for adjusting the composition of silver-palladiumlayers. Furthermore, these reducing agents contribute to improving thelayer appearance and to increasing the luminance (L value, CIE Lab) ofthe deposited layers. The present invention also discloses a method forthe electrolytic deposition of silver-rich silver-palladium alloys.

Electrical contacts are used today in virtually all electricalappliances. Their applications range from simple plug connectors tosafety-relevant, sophisticated switching contacts in the communicationssector, for the automotive industry or for aerospace technology. Herethe contact surfaces are required to have good electrical conductivity,low contact resistance with long-term stability, as well as goodcorrosion and wear resistance with insertion forces that are as low aspossible. In electrical engineering, plug contacts are often coated witha hard-gold alloy layer, consisting of gold-cobalt, gold-nickel orgold-iron. These layers have a good resistance to wear, a goodsolderability, a low contact resistance with long-term stability, andgood corrosion resistance. Due to the rising price of gold, lessexpensive alternatives are being sought.

As a substitute for hard-gold plating, coating with silver-rich silveralloys (hard silver) has proven advantageous. Silver and silver alloysare amongst the most important contact materials in electricalengineering, not just on account of their high electrical conductivityand good oxidation resistance. These silver-alloy layers have, dependingon the metal that is added to the alloy, layer properties similar tothose of currently used hard-gold layers and layer combinations, such aspalladium-nickel with gold flash. In addition, the price for silver isrelatively low compared with other precious metals, in particularhard-gold alloys.

One constraint on the use of silver is, for example, the fact that inatmospheres containing sulfur or chlorine silver has lower corrosionresistance than hard gold. Apart from the visible surface change,tarnishing films of silver sulfide in most cases do not represent anygreat danger since silver sulfide is semi-conductive, soft, and iseasily wiped away during the insertion process provided contact forcesare strong enough. Tarnishing films of silver chloride, on the otherhand, are non-conductive, hard and not easily displaced. A relativelyhigh proportion of silver chloride in the tarnishing layer thus leads toproblems with the contact properties (literature: Marjorie Myers:Overview of the use of silver in connector applications; Interconnect &Process Technology, Tyco Electronics, Harrisburg, February 2009).

U.S. Pat. No. 3,980,531 discloses a cyanide-free electrolyte for thegalvanic deposition of alloys containing gold, silver and/or palladium.The baths contain a thiosulfate, a sulfite and a borate or phosphate.The alloys are deposited in the weakly acidic to highly alkaline pHrange. The electrolyte can optionally include the salts of base metals,such as arsenic or cadmium. Deposition takes place at current densitiesof 0.1 to 5 A/dm². In the baths in accordance with U.S. Pat. No.3,980,531, the composition of the deposited alloy depends on theconcentrations of the metal salts used and the current density used. Theappearance of the alloys varies from matt to high gloss. Due to the useof arsenic and cadmium, this electrolyte is no longer acceptable todayon account of existing regulations (REACH).

U.S. Pat. No. 6,251,249 B1 discloses electrolytes for the deposition ofprecious metals onto solid substrates. These electrolytes areiodide-free and contain the precious metal to be deposited in the formof alkane sulfonates, alkane sulfonamides and/or alkane sulfonimides. Inaddition, the electrolytes contain an organosulfur compound and/or acarboxylic acid. The precious metals are preferably deposited in atemperature range of 20° C. to 60° C. The pH value can be between 0 and12. The electrolytes are suitable for electroless and electrolyticdeposition of precious metal layers, as well as for immersion plating.The examples in U.S. Pat. No. 6,251,249 B1 relate exclusively toimmersion plating, and either silver or palladium is deposited, but nosilver-palladium alloy. No information is provided about theelectrolytic deposition of silver-palladium alloys or about theircomposition.

In EP 0 065 100 A1 a galvanic palladium electrolyte is described whichcontains palladium sulfite and an acid. The electrolyte containssulfuric acid and/or phosphoric acid and can be used at 20° C. to 40° C.80 to 95% of the palladium content can be added as palladium sulfate,the rest as palladium sulfite. However, EP 0 065 100 A21 is silent aboutthe deposition of palladium alloys.

DE 10 2013 215 476 B3 discloses a cyanide-free, acidic and aqueouselectrolyte for the deposition of silver-palladium alloys. In additionto silver and palladium salts, the electrolyte contains a selenium ortellurium compound, urea and/or at least one amino acid and a sulfonicacid. With this electrolyte, silver-palladium alloys with apredominantly silver content can be deposited across a wide currentdensity range. However, only semi-matt alloy coatings can be producedwith this electrolyte. With increasing current density, the layersproduced exhibit a distinct brownish tinge. At the same time theelectrolyte shows a marked dependence of the alloy composition on thecurrent density applied. The alloy can only be influenced by shiftingthe concentration of the alloying metals or by varying the temperatureof the electrolyte during deposition.

The electrolytes known from prior art for the electrolytic deposition ofsilver-palladium alloys do not allow the deposition of silver-palladiumalloys which, over a wide current density range, are not only highlyglossy but also have a constant ratio of silver to palladium. Similarly,the alloy composition can only be adjusted to a very limited extent byshifting the bath parameters. In previously known baths the palladiumcontent in the deposited layers decreases as current density rises. Theappearance of the deposited layers changes at the same time: as thecurrent density increases, the layers take on an increasingly markedbrownish tinge. Inhomogeneities in the layer, such as haze and speckles,increase at the same time.

Despite the large number of electrolytes already known for theelectrolytic deposition of silver-palladium alloys, there is, therefore,still a need for electrolytes which in practical use are superior toprior-art electrolytes. Such electrolytes should be stable enough forindustrial use and permit the deposition of stable and bright alloycompositions over the widest possible current-density range. Simpleadjustment of the alloy composition is equally important. Theelectrolytes should remain fully functional even after a high currentdensity load, and the layers deposited with these electrolytes should behomogeneous and advantageous with regard to use in contact materials.The composition of the deposited alloy is especially advantageously 90±3wt % silver, 10±3 wt % palladium and 0-3 wt % tellurium and/or selenium.

These and other problems obviously arising for the person skilled in theart from the closest pertinent prior art are solved by an electrolyteaccording to the present claim 1. Protection is sought for furtherpreferred embodiments in the subordinate claims dependent on claim 1.Claim 9 relates to a preferred method for the deposition ofsilver-palladium alloys in which the electrolyte according to theinvention is used. Claims 10 to 12 relate to preferred embodiments ofthe present process.

The problem of providing a cyanide-free, acidic and aqueous electrolytefor the electrolytic deposition of bright silver-palladium alloys with apredominantly silver content is solved according to the invention by anaqueous electrolyte which in its dissolved form contains the followingcomponents:

-   -   a) a silver compound in a concentration of 1-300 g/l silver;    -   b) a palladium compound in a concentration of 0.1-100 g/l        palladium;    -   c) a tellurium and/or selenium compound in a concentration of        0.002-10 g/l tellurium and/or selenium, based on the total        amount of tellurium and selenium in the electrolyte;    -   d) a compound selected from the group consisting of urea, urea        derivatives, thiourea and thiourea derivatives and mixtures        thereof in a concentration of 0.05-2 mol/l, based on the total        amount of urea, urea derivatives, thiourea and thiourea        derivatives in the electrolyte and/or one or more amino acids        selected from the group consisting of alanine, aspartic acid,        cysteine, glutamine, glutamic acid, glycine, lysine, leucine,        methionine, phenylalanine, phenylglycine, proline, serine,        tyrosine and valine in a concentration of 0.005-0.5 mmol/l,        based on the total amount of amino acids in the electrolyte;

e) at least one sulfonic acid in a concentration of 0.25-4.75 mol/l,based on the total amount of sulfonic acids,

-   -   f) at least one reducing agent selected from the group of formic        acid, oxalic acid, ascorbic acid, hydrazine,        hexamethylenetetramine, salts and/or esters of sulfurous acid,        gaseous sulfites, sulfinic acids and their salts and/or esters,        formaldehyde, sodium formaldehyde sulfoxylate, benzaldehyde,        benzaldehyde derivatives, hydroxybenzenes and their esters,        polyphenols and their esters, phenolsulfonic acids and their        salts and/or esters, and glutathione and also its salts and/or        esters in a concentration of 0.1 mmol/l-1 mol/l, based on the        total amount of these reducing agents.

Surprisingly, it was found that with the electrolytes described here itis possible over a wide current density range to deposit on conductivesubstrates homogeneous and bright silver-palladium alloy layers whichare eminently suitable for use in contact materials. As a result, theelectrolytes according to the invention are suitable as a substitute forhard-gold alloys in contact materials. At the same time, the palladiumcontent in the layer as a function of the amount of the reducing agentadded can be simply adjusted by means of the reducing agents(brighteners) added. With increasing concentration of the reducingagents the palladium content of the deposited layer increases. Theelectrolyte according to the invention here shows comparatively highstability, which makes it appear especially advantageous in industrialapplication. With the present electrolytes, high-quality electricalcontact materials can be advantageously produced even in rack andhigh-speed coating systems. The electrolyte preferably contains only theabove constituents.

The electrolyte according to the invention can be used in a currentdensity range of 0.1 to 100 A/dm². A current density range of 0.5 to 20A/dm² is preferred.

In the present invention ‘homogeneous’ silver-palladium alloy coatingsmeans such layers whose appearance is uniform as regards color and layerproperties. Layer properties in this case are gloss, brightness,hardness, and corrosion resistance. The silver-palladium alloy layersare here homogeneous in two respects. Firstly, the silver-palladiumalloy layer deposited on a particular electrically conductive substrateis homogeneous according to the above definition. Secondly, theappearance of the deposited silver-palladium alloys is homogeneous whenlayers are deposited on a plurality of identical electrically conductivesubstrates with different current densities from the same electrolyte,without change in the composition of the electrolyte, in temperature orin movement, said layers having an identical alloy composition and anidentical visual appearance—in other words, the deposited layers are inthis case homogeneous irrespective of the current density.

It is known to the person skilled in the art that the color andbrightness of metallic coatings can be determined with the aid of theso-called L*a*b* measurement according to CIEL*a*b (www.cielab.de),wherein the L* value indicates brightness. The brightness values (L*values) of the silver-palladium alloy layers according to the inventionlie between 80 and 90 L*a*b* (measuring instrument X-Rite SP62,illuminant D65/10).

Gloss can be assessed by measuring the reflectivity. In thesilver-palladium alloy layers according to the invention the addition ofthe reducing agents causes a rise in reflectivity of 5-40% of theinitial value depending on the current density applied and theconcentration of the reducing agent. The reflectivity was measured withthe BYK-Gardner micro-TRI-gloss meter. Measurement was carried out at a20° angle of incidence and a 20° angle of reflection of the light beamaccording to EN ISO 7668. Measurement of the gloss of surfaces is knownto the person skilled in the art and information in this regard may befound in, for example, ‘Schriftenreihe Galvanotechnik undOberflächenbehandlung. Prüfung von funktionellen metallischen Schichten[Publication series: Electroplating and surface treatment: Inspectingfunctional metal coatings], Section 4.3: Glanz-und Reflexionsmessung anOberflächen’ [Gloss and reflection measurement of surfaces], Eugen GLeuze-Verlag, Saulgau, 1st ed. 1997, pp. 117-125.

Galvanic baths are solutions containing metal salts from whichelectrochemically metallic precipitates (coatings) can be deposited onsubstrates (objects). Galvanic baths of this kind are often also termed‘electrolytes’. Accordingly, the cyanide-free and aqueous galvanic bathsaccording to the invention are hereinafter referred to as‘electrolytes’.

The electrolyte according to the invention for the electrolyticdeposition of bright homogeneous silver-palladium alloys with apredominant content of silver, and also the method for depositing suchsilver-palladium alloys are explained below, wherein the inventioncomprises all embodiments listed below, either individually or incombination with each other.

The person skilled in the art will generally be familiar with the metalcompounds which can be added to the electrolyte.

The silver compound contained in the electrolyte according to theinvention is preferably a silver salt which is soluble in thiselectrolyte. The silver salts are here preferably selected from thegroup consisting of silver methanesulfonate, silver carbonate, silversulfate, silver phosphate, silver pyrophosphate, silver nitrate, silveroxide, silver lactate, silver fluoride, silver bromide, silver chloride,silver iodide, silver azide, silver sulfide and silver sulfate. Silvernitrate, silver carbonate, silver methanesulfonate, silver chloride andsilver oxide are particularly preferably used in the electrolyteaccording to the invention. Here the person skilled in the art should beguided by the principle that as few additional substances as possibleshould be added to the electrolyte. For this reason, the person skilledin the art will give the utmost preference to selecting silvermethanesulfonate, silver carbonate or silver oxide as the silver salt tobe added. As regards the concentration of the silver compound employed,the person skilled in the art should be guided by the limit values givenabove. The silver compound in a concentration of 1-300 g/l of silver ispreferable, more preferable is 2-100 g/l of silver and most preferableis between 4-15 g/l of silver in the electrolyte.

The palladium compound to be employed is preferably also a salt solublein the electrolyte or a soluble complex. The palladium compound usedhere is preferably selected from the group consisting of palladiumhydroxide, palladium chloride, palladium sulfate, palladiumpyrophosphate, palladium nitrate, palladium phosphate, palladiumbromide, palladium P salt (diammine dinitrito palladium (II); ammoniacalsolution), palladium glycinates, palladium acetates, tetramminepalladium(II) chloride, tetramminepalladium (II) bromide, palladiummethanesulfonate, diamminedinitropalladium (II) chloride,diamminedinitropalladium (II) bromide, diamminedinitropalladium (II)sulfate, potassium di-oxalatopalladate, palladium iodide,tetramminepalladium (II) sulfate, bis (ethylenediamino) palladium (II)bromide, bis (acetylacetonato) palladium (II), diamminedichloropalladium (II), palladium oxide hydrate, tetramminepalladium(II) hydrogen carbonate, bis(ethylenediamine) palladium (II) chloride,bis(ethylenediamine) palladium (II) sulfate and bis(ethylenediamine)palladium (II) carbonate. The palladium compound is advantageouslyselected from palladium hydroxide, palladium chloride, palladiumglycinate, palladium methanesulfonate and palladium sulfate.

The palladium compound is in this case added to the electrolyte in aconcentration as indicated above. The palladium compound is preferablyused in a concentration of 0.1 to 100 g/l palladium, most preferably ina concentration of 2-20 g/l palladium in the electrolyte.

The electrolyte according to the invention is aqueous. The silver andpalladium compounds to be employed are preferably salts soluble in theelectrolyte or soluble complexes. The terms ‘soluble salt’ and ‘solublecomplex’ therefore refer to such salts and complexes as dissolve in theelectrolyte at the working temperature. Here the working temperature isthat temperature at which the silver-palladium alloy is deposited. Inthe context of the present invention, a substance is deemed soluble whenat least 0.002 g/l of this substance dissolves in the electrolyte at theworking temperature.

The deposited alloys, which contain silver, palladium and also seleniumand/or tellurium, here have a composition comprising 70-99 wt % silver,1-30 wt % palladium and 0.1-5 wt % selenium and/or tellurium. Theproportions of silver, palladium and selenium and/or tellurium here addup to 100 wt %. According to the invention the concentrations in theelectrolyte of the metals to be deposited are set within the frameworkgiven above in such a way that the result is a silver-rich alloy. Itshould be noted that not only does the concentration of the metals to bedeposited have an influence on the silver concentration and brightnessof the deposited alloy but so also do the current density set, thequantity of tellurium compound and/or selenium compound used, and theaddition of the reducing agents. The person skilled in the art will knowhow the corresponding parameters must be set in order to obtain thetarget alloy desired, or will be able to determine this by routineexperimentation. Efforts are preferably made to obtain an alloy in whichsilver has a concentration of 70-99 wt %, more preferably 80-95 wt % andmost preferably 87-94 wt %. The palladium content of the alloysaccording to the invention is 1-30 wt %, preferably 5-20 wt % andparticularly preferably 6-13 wt %. The selenium or tellurium content ofthe alloy according to the invention is 0.1-5 wt %, preferably 0.5-4 wt% and particularly preferably 1-3 wt %.

The alloys according to the invention which contain silver, palladiumand also selenium and/or tellurium are hereinafter referred to as‘silver-palladium alloys’.

The selenium or tellurium compound which is used in the electrolyte canbe appropriately selected by the person skilled in the art within theframework of the concentrations indicated above. A concentration of0.002-10 g/l tellurium and/or selenium can be selected as the preferredconcentration range and a concentration of 0.1-5 g/l tellurium and/orselenium as the most preferred range. The concentration data here relateto the total amount of tellurium and selenium in the electrolyte.Suitable selenium and tellurium compounds are those in which selenium ortellurium is present in oxidation states +4 or +6. Selenium andtellurium compounds are advantageously used in the electrolyte in whichselenium or tellurium in oxidation state +4 is present. The selenium andtellurium compounds are particularly preferably selected fromtellurites, selenites, tellurous acid, selenious acid, telluric acid,selenium acid, selenocyanates, tellurocyanates and selenate andtellurate. The use of tellurium compounds rather than selenium compoundsis generally preferred here. More particularly preferable is theaddition of tellurium to the electrolyte in the form of a salt of thetellurous acid in, for example, the form of potassium tellurite.

The electrolyte according to the invention contains a compound selectedfrom the group consisting of urea, urea derivatives, thiourea, thioureaderivatives and mixtures thereof and/or one or more α-amino acids whichserve as complexing agents for the palladium and contribute toincreasing the stability of the present electrolyte.

Urea derivatives are selected from dimethyl urea, ethylene urea, N,N′-dimethyl propylene urea, and N-(2-hydroxyethyl) ethylene urea. Thethiourea derivatives are, for example, 3-S-isothiuronium propanesulfonate and n-ethyl thiourea.

In one advantageous embodiment, component (d) of the electrolyteaccording to the invention, that is, the complexing agent for thepalladium, is urea.

The one or more α-amino acids are here selected from the groupconsisting of alanine, aspartic acid, cysteine, glutamine, glutamicacid, glycine, lysine, leucine, methionine, phenylalanine,phenylglycine, proline, serine, tyrosine and valine. Preferably theamino acids used here are those which have only alkyl groups in thevariable residue. In one advantageous embodiment, the α-amino acid isselected from alanine, glycine and valine. The use of glycine and/oralanine is most preferable.

Urea, urea derivatives, thiourea, thiourea derivatives and mixturesthereof are used in a concentration of 0.05 to 2 mol/l, preferably 0.2to 1.5 mol/l, based on the total amount of urea and urea derivatives inthe electrolyte. The concentration of the one or more α-amino acids inthe electrolyte according to the invention is here 0.005 to 0.5 mol/l,preferably 0.01-0.2 mol/l. In the case of α-amino acids, theseconcentration data refer to the total amount of α-amino acid or α-aminoacids, regardless of whether the electrolyte contains one or moreα-amino acids.

Within the concentration framework given above the person skilled in theart can freely select the optimum concentration for the amino acid used.He will be guided by the fact that if the quantity of amino acid is toosmall it will not produce the stabilizing effect desired, while too higha concentration can inhibit the deposition of palladium.

The electrolyte according to the invention is used within an acidic pHrange. Optimal results can be obtained with pH values of <2 in theelectrolyte. The person skilled in the art will know how he can set thepH value of the electrolyte. He will be guided by the idea ofintroducing as few additional substances into the electrolyte aspossible which could adversely affect the deposition of the alloy inquestion. In one most preferable embodiment the pH value is determinedsolely by the addition of sulfonic acid. This then preferably yieldsstrongly acidic deposition conditions where the pH value is less than 1and possibly may even reach 0.1 or even in marginal cases 0.01. In theoptimum case the pH value will be 0.3-0.6.

In the electrolyte according to the invention, at least one sulfonicacid is used in addition in a concentration of 0.25-4.75 mol/l, whereinthe concentration is based on the total amount of sulfonic acids used.The concentration is preferably 0.5-3 mol/1 and most preferably 0.8-2.0mol/l. The at least one sulfonic acid serves firstly to establish anappropriate pH value in the electrolyte. Secondly, its use leads to afurther stabilization of the electrolyte according to the invention. Theupper limit of the sulfonic acid concentration is due to fact that attoo high a concentration only silver is deposited. In principle thesulfonic acids known to the person skilled in the art for use inelectroplating technology can be used. Sulfonic acids are preferablyselected from the group consisting of ethanesulfonic acid,propanesulfonic acid, benzenesulfonic acid, and methanesulfonic acid.Here they can be used individually or as mixtures. Propanesulfonic acidand methanesulfonic acid are more particularly preferred in thiscontext. Most particularly preferred is methanesulfonic acid.

The at least one reducing agent is selected from formic acid, oxalicacid, ascorbic acid, hydrazine, hexamethylenetetramine, salts and/oresters of sulfurous acid, gaseous sulfites, sulfinic acids and theirsalts and/or esters, formaldehyde, sodium formaldehyde sulfoxylate,benzaldehyde, benzaldehyde derivatives, hydroxybenzenes and theiresters, polyphenols and their esters, phenolsulfonic acids and theirsalts and/or esters, and glutathione and also its salts and/or esters.

In one advantageous embodiment, the reducing agent is selected fromhydroxybenzolenes, sodium formaldehyde sulfoxylate and ascorbic acid.

In another advantageous embodiment, the reducing agent is selected fromsalts and/or esters of sulfurous acid.

The salts of sulfurous acid can be sulfites or hydrogen sulfites. Thesulfites and hydrogen sulfites are advantageously lithium, sodium,potassium, or ammonium salts.

The esters of sulfurous acid are compounds with the general formulaR1-O—S(═O)—O—R2, where R1 and R2 are independently selected from linearor branched acyclic alkyl groups with 1 to 10 carbon atoms, cyclic alkylgroups with 3 to 10 carbon atoms, aryl groups and benzyl groups. Withinthe context of the present invention the linear or branched acyclicalkyl groups with 1 to 10 carbon atoms are selected from methyl, ethyl,n-propyl, isopropyl, 1-butyl, 2-butyl, tert-butyl, 1-pentyl, 2-pentyl,3-pentyl, 3-methylbutyl, 2,2-dimethylpropyl and all isomers of hexyl,heptyl, octyl, nonyl, and decyl. It is known to the person skilled inthe art that cyclic alkyl groups must contain at least three carbonatoms. In the context of the present invention, cyclic alkyl groups willadvantageously include propyl, butyl, pentyl, hexyl, heptyl and octylrings. A cyclic alkyl group for the purpose of the present invention isselected from the aforenamed cyclic alkyl groups which carry no othersubstituents, and from the aforenamed cyclic alkyl groups which fortheir part are bound to one or more acyclic alkyl groups. In the lattercase, the cyclic alkyl group can according to the above formula be boundto the oxygen atom via a cyclic or an acyclic carbon atom of the cyclicalkyl group. According to the definition given above of the term ‘alkylgroup’, cyclic alkyl groups also contain a maximum of 10 carbon atoms.If, in the case of groups R1 and R2, an aryl group is concerned, thiswill be selected from phenyl, naphthyl and anthracenyl.

In the case of gaseous sulfites, the gas which is introduced into theelectrolyte is SO₂.

The sulfinic acids are compounds with the general formula R3-S(═O)—OH,where R3 is a linear or branched acyclic alkyl group with 1 to 10 carbonatoms, a cyclic alkyl group with 3 to 10 carbon atoms, an aryl or benzylgroup and where these groups are defined as described above for R1 andR2.

Benzaldehyde derivatives are selected from benzaldehyde sulfonic acid,its salts and esters, for example, benzaldehyde-2-sulfonic acid sodiumsalt, dimethylaminobenzaldehyde, 3-chlorobenzaldehyde,4-chlorobenzaldehyde, 2-methoxybenzaldehyde, 2-methyl benzaldehyde,2-nitrobenzaldehyde, 3,5-dibromobenzaldehyde, 3-nitrobenzaldehyde and3.5-dimethoxybenzaldehyde.

Hydroxybenzenes are selected from phenol, catechol, resorcinol,hydroquinone, pyrogallol, hydroxyquinone and phloroglucinol.

If the at least one reducing agent is a salt of an organic compound, asodium, potassium, lithium or ammonium salt will be advantageouslyselected. In the case of organic acids with multiple protons, a single,several or all acidic hydrogen atoms can be replaced by sodium,potassium, lithium, or ammonium ions. If more than one acidic hydrogenatom is replaced by sodium, potassium, lithium, or ammonium ions, thesecations can be identical or different.

In the case of the at least one reducing agent, this may also be anester of an organic compound. It is known to the person skilled in theart that esters are the condensation products of an alcohol and acarboxylic acid. Esters of the alcohols in the list given above ofsuitable reducing agents are therefore a condensation product of one ofthe aforementioned alcohols and a carboxylic acid R4-COOH, and esters ofcarboxylic acids in the list given above are condensation products ofone of the aforementioned carboxylic acids with an alcohol R5-OH.

Here R4 and R5 are selected from linear or branched acyclic alkyl groupswith 1 to 10 carbon atoms, cyclic alkyl groups with 3 to 10 carbonatoms, and aryl or benzyl groups, where these groups are defined asdescribed above for R1 and R2.

Particularly advantageously the at least one reducing agent is selectedfrom salts or esters of sulfurous acid and gaseous sulfites.

The at least one reducing agent is contained in the electrolyte in aconcentration of 1-100 mmol/l, advantageously in a concentration of 5-30mmol/l, wherein the concentration is based on the total amount of theaforementioned reducing agents in the electrolyte.

The electrolyte according to the invention furthermore contains at leastone sulfonic acid in a concentration of 0.25-4.75 mol/l. Theconcentration is preferably 0.5-3 mol/1 and most preferably 0.8-2.0mol/l. The at least one sulfonic acid serves firstly to establish anappropriate pH value in the electrolyte. Secondly, its use leads to afurther stabilization of the electrolyte according to the invention. Theupper limit of the sulfonic acid concentration is due to the fact thatat too high a concentration only silver is deposited. The sulfonic acidshave the general molecular formula R6-S(═O)2-OH, wherein R6 represents alinear or branched acyclic alkyl group with 1 to 10 carbon atoms, acyclic alkyl group with 3 to 10 carbon atoms, or an aryl or benzylgroup, wherein these groups are defined as described above for R1 andR2. Sulfonic acids are preferably selected from the group consisting ofmethanesulfonic acid, ethanesulfonic acid, propanesulfonic acid andbenzenesulfonic acid. Methanesulfonic acid and propanesulfonic acid aremore particularly preferred in this context. Most particularly preferredis methanesulfonic acid.

Optionally, the electrolyte according to the invention can additionallycontain a surfactant. This surfactant is selected from anionic andnon-ionic surfactants. Examples include polyethylene glycol adducts,fatty alcohol sulfates, alkyl sulfates, alkyl sulfonates, arylsulfonates, alkylaryl sulfonates and heteroarylsulfonates, betaines,fluorosurfactants and their salts and derivatives. Suitable surfactantsare known to the person skilled in the art, for example, as in N.Kanani: Galvanotechik [Electroplating], Hanser-Verlag, Munich andVienna, 2000, pp. 84 ff. Before the addition of a surfactant theelectrolyte according to the invention has a surface tension greaterthan or equal to 70 mN/m. If a surfactant is added, its concentrationwill be advantageously chosen such that the surface tension of theelectrolyte decreases to a value less than or equal to 50 mN/m. Thesurface tension can be measured with a bubble pressure tensiometer.

In a further embodiment, the present invention relates to a method forthe electrolytic deposition of silver-palladium alloys with apredominant content of silver from an electrolyte according to theinvention, wherein an electrically conductive substrate is immersed inthe electrolyte and a flow of current established between an anode incontact with the electrolyte and the substrate as cathode. It should benoted that the embodiments mentioned as preferable for the electrolyteapply mutatis mutandis to the method addressed here.

The temperature prevailing during the deposition of the silver-palladiumalloy can be selected as desired by the person skilled in the art. Hewill be guided on the one hand by an adequate deposition rate and anapplicable current density range and on the other hand by cost aspectsor the stability of the electrolyte. A temperature of 25° C. to 75° C.is set advantageously in the electrolyte, preferably between 30° C. and65° C. The use of the electrolyte at a temperature between 45° C. and55° C. appears more particularly preferable.

The current density which is established in the electrolyte between thecathode and the anode during the deposition process can be selected bythe person skilled in the art according to the efficiency and quality ofdeposition. Depending on the application and coating plant type, thecurrent density in the electrolyte is advantageously set to 0.1 to 100A/dm². If necessary, current densities can be increased or reduced byadjusting the system parameters, such as the design of the coating cell,flow rates, the anode or cathode set-ups, and so on. A current densityof 0.5-20 A/dm² is advantageous, 1-20 A/dm² is preferable, and 1.5-15A/dm² most preferable.

As has already been indicated, the electrolyte according to theinvention is an acidic type. The pH value should preferably be <2, andparticularly preferably <1. It may be the case that fluctuations occurin the pH value of the electrolyte during electrolysis. In one preferredembodiment of the present method the person skilled in the art willtherefore take steps to monitor the pH value during electrolysis and, ifnecessary, adjust it to the setpoint value.

Various anodes can be employed when using the electrolyte. Soluble orinsoluble anodes are just as suitable as the combination of soluble andinsoluble anodes. If a soluble anode is used, a silver anode isparticularly preferred.

As insoluble anodes preference is given to those made of a materialselected from the group consisting of platinized titanium, graphite,iridium-transition metal mixed oxide and special carbon material (DLC ordiamond-like carbon) or combinations of these anodes. Particularlypreferred for implementation of the invention are mixed-oxide anodescomposed of iridium-ruthenium mixed oxide, iridium-ruthenium-titaniummixed oxide or iridium-tantalum mixed oxide. More particularly preferredare platinum-titanium anodes. More information may be found in Cobley,A. J et al. (The use of insoluble anodes in acid sulphate copperelectrodeposition solutions, Trans IMF, 2001, 79(3), pp. 113 and 114).

The present invention presents a silver-palladium alloy electrolyte withan added reducing agent for alloy adjustment and as a brightener andalso for the electrolytic deposition of silver-palladium layers, and acorresponding method. The electrolyte contains at least one reducingagent for alloy adjustment and brightening: by adding the at least onereducing agent the palladium content of the deposited silver-palladiumalloy can be adjusted. As already indicated above in the text, thealloys deposited according to the invention have a compositioncomprising 70-99 wt % silver, 1-30 wt % palladium and 0.1-5 wt %selenium and/or tellurium, wherein the proportions of silver, palladiumand selenium and/or tellurium add up to 100 wt %. In addition, theelectrolyte according to the invention leads to a more homogeneousdeposition in comparison with conventional silver-palladium alloyelectrolytes.

Layers deposited from conventional silver-palladium electrolytes have,depending on the current density applied, L* values of 67-78. With thenew electrolyte system according to the invention, markedly higher L*values are achieved for the deposited layers which are also uniform overthe current density range applied. These values lie between 80 and 90,depending on the reducing agent used.

Against the background of the known prior art, this was not to beexpected.

EMBODIMENTS

Various basic electrolytes were prepared and in each case a reducingagent in two different concentrations added. From these electrolytes,both with and without a reducing agent, silver-palladium layers werethen deposited, characterized and compared to each other.

Embodiment 1

Basic Electrolyte:

100 ml/l methanesulfonic acid 70%

3 g/l glycine

10 g/l palladium (as palladium hydroxide)

5 g/l silver (as silver nitrate)

0.5 g/l tellurium (as tellurous acid)

Reducing Agents:

-   -   0 g/l sodium formaldehyde sulfoxylate    -   0.95 g/l sodium formaldehyde sulfoxylate (8 mmol)    -   7.1 g/l sodium formaldehyde sulfoxylate (40 mmol)

Temperature: 30° C.

Anodes: PtTi

The palladium content of the deposited layers was measured using anX-ray fluorescence analysis method (XRF) (Fischerscope XDV-SDD, softwareWIN-FTM Version 6.28-S-PDM).

Measurement Results for Palladium Content:

Sodium formaldehyde sulfoxylate content [g/l] Current density [A/dm²] Pdcontent [wt %] 0 1 4.2 0 2 3.2 0 3 3.0 0.95 1 5.7 0.95 2 3.5 0.95 3 3.44.7 1 9.1 4.7 2 6.8 4.7 3 5.4

FIG. 1 shows the results of palladium content measurement.

The brightness of the deposited layers was measured in the form of theL* value according to CIEL*a*b.

Measurement Results:

Sodium formaldehyde sulfoxylate content [g/l] Current density [A/dm²]Brightness [L*] 0 1 78.3 0 2 73.4 0 3 73.0 0.95 1 73.6 0.95 2 83.0 0.953 80.5 4.7 1 75.6 4.7 2 77.2 4.7 3 78.8

Embodiment 2

Basic Electrolyte:

80 ml/l methanesulfonic acid 70%

5 g/l urea

10 g/l palladium (as palladium chloride)

6 g/l silver (as silver methanesulfonate)

1.0 g/l tellurium (as potassium tellurite)

Reducing Agents:

-   -   0 g/l ascorbic acid    -   0.14 g/l ascorbic acid    -   0.42 g/l ascorbic acid

Temperature: 60° C.

Anodes: PtTi

The palladium content of the deposited layers was measured using anX-ray fluorescence analysis method (XRF).

Measurement Results for Palladium Content:

Ascorbic acid content [g/l] Current density [A/dm²] Pd content [wt %] 01 3.8 0 2 2.9 0 3 2.7 0.14 1 4.2 0.14 2 3.1 0.14 3 2.7 0.42 1 5.3 0.42 23.6 0.42 3 3.3

FIG. 2 shows the results of palladium content measurement.

The brightness of the deposited layers was measured in the form of theL* value according to CIEL*a*b.

Measurement Results for Brightness:

Ascorbic acid content [g/l] Current density [A/dm²] Brightness [L*] 0 181.8 0 2 67.9 0 3 64.5 0.14 1 83.6 0.14 2 76.6 0.14 3 71.0 0.42 1 83.00.42 2 79.0 0.42 3 73.6

Embodiment 3

Basic Electrolyte:

100 ml/l methanesulfonic acid 70%

5 g/l valine

12 g/l palladium (as palladium hydroxide)

25 g/l silver (as silver nitrate)

1.5 g/l tellurium (as tellurous acid)

Reducing Agents:

-   -   0 g/l hydroquinone    -   0.5 g/l hydroquinone    -   1 g/l hydroquinone

Temperature: 60° C.

Anodes: Graphite

The palladium content of the deposited layers was measured using anX-ray fluorescence analysis method (XRF).

Measurement Results for Palladium Content:

Hydroquinone content [g/l] Current density [A/dm²] Pd content [wt %] 0 11.4 0 2 2.9 0 3 2.8 0.5 1 6.8 0.5 2 5.5 0.5 3 6.0 1.0 1 16.8 1.0 2 15.01.0 3 14.4

FIG. 3 shows the results of palladium content measurement.

The brightness of the deposited layers was measured in the form of theL* value according to CIEL*a*b.

Measurement Results for Brightness:

Hydroquinone content [g/l] Current density [A/dm²] Brightness [L*] 0 181.7 0 2 77.8 0 3 72.5 0.5 1 83.1 0.5 2 81.6 0.5 3 77.1 1.0 1 76.5 1.0 277.7 1.0 3 73.8

Embodiment 4

Basic Electrolyte:

200 ml/l methanesulfonic acid 70%

2 g/l glycine

15 g/l palladium (as palladium sulfate)

8 g/l silver (as silver carbonate)

0.5 g/l tellurium (as tellurous acid)

Reducing Agents:

-   -   0 g/l sodium sulfite    -   1 g/l sodium sulfite    -   2 g/l sodium sulfite

Temperature: 40° C.

Anodes: PtTi

The palladium content of the deposited layers was measured using anX-ray fluorescence analysis method (XRF).

Measurement Results for Palladium Content:

Sodium sulfite content [g/l] Current density [A/dm²] Pd content [wt %] 01 6.2 0 2 4.9 0 3 3.5 1.0 1 10.0 1.0 2 8.1 1.0 3 8.1 2.0 1 15.6 2.0 212.3 2.0 3 11.7

FIG. 4 shows the results of palladium content measurement.

The brightness of the deposited layers was measured in the form of theL* value according to CIEL*a*b.

Measurement Results for Brightness:

Sodium sulfite content [g/l] Current density [A/dm²] Brightness [L*] 0 180.0 0 2 76.3 0 3 71.1 1.0 1 81.8 1.0 2 82.2 1.0 3 81.2 2.0 1 78.4 2.0 277.8 2.0 3 78.3

1. Cyanide-free, acidic and aqueous electrolyte for the electrolyticdeposition of bright silver-palladium alloys with a predominantly silvercontent which in its dissolved form contains the following components:a) a silver compound in a concentration of 1-300 g/l silver; b) apalladium compound in a concentration of 0.1-100 g/l palladium; c) atellurium and/or selenium compound in a concentration of 0.002-10 g/ltellurium and/or selenium, based on the total amount of tellurium andselenium in the electrolyte; d) urea and/or urea derivatives in aconcentration of 0.05-1.5 mol/l, based on the total amount of urea andurea derivatives in the electrolyte and/or one or more amino acids,selected from the group consisting of alanine, aspartic acid, cysteine,glutamine, glutamic acid, glycine, lysine, leucine, methionine,phenylalanine, phenylglycine, proline, serine, tyrosine and valine in aconcentration of 0.005-0.5 mol/l, based on the total amount of aminoacids in the electrolyte; e) at least one sulfonic acid in aconcentration of 0.25-4.75 mol/l, based on the total amount of sulfonicacids; f) at least one reducing agent selected from the group of formicacid, oxalic acid, ascorbic acid, hydrazine, hexamethylenetetramine,salts and/or esters of sulfurous acid, gaseous sulfites, sulfinic acidsand their salts and/or esters, formaldehyde, sodium formaldehydesulfoxylate, benzaldehyde, benzaldehyde derivatives, hydroxybenzenes andtheir esters, polyphenols and their esters, phenolsulfonic acids andtheir salts and/or esters, and glutathione and also its salts and/oresters in a concentration of 1-100 mmol/l, based on the total amount ofthese reducing agents.
 2. Electrolyte according to claim 1, wherein thesilver compound is selected from silver nitrate, silver carbonate,silver methanesulfonate, silver chloride and silver oxide, 3.Electrolyte according to claim 1, wherein the palladium compound isselected from palladium hydroxide, palladium chloride, palladiumglycinate, palladium methanesulfonate and palladium sulfate. 4.Electrolyte according to claim 1, wherein the selenium and/or telluriumcompounds are selected from tellurites, selenites, tellurous acid,selenious acid, telluric acid, selenate and also tellurate. 5.Electrolyte according to claim 1, wherein the α-amino acid is selectedfrom alanine, glycine and valine.
 6. Electrolyte according to claim 1,wherein component (d) is urea.
 7. Electrolyte according to claim 1,wherein the at least one sulfonic acid is selected from ethanesulfonicacid, propanesulfonic acid, benzenesulfonic acid, and methanesulfonicacid.
 8. Electrolyte according to claim 1, wherein the at least onereducing agent is selected from hydroxyphenols, ascorbic acid and saltsand/or esters of sulfurous acid.
 9. Method for the electrolyticdeposition of silver-palladium layers predominantly containing silverfrom an electrolyte according to claim 1, wherein an electricallyconductive substrate is immersed in the electrolyte and a flow ofcurrent established between an anode in contact with the electrolyte andthe substrate as cathode.
 10. Method according to claim 9, wherein theelectrolyte temperature is 25 to 70° C.
 11. Method according to claim 9,wherein the current is between 0.5 and 20 A/dm² during electrolysis. 12.Method according to claim 9, wherein the pH value is set to a constantvalue of <2 during electrolysis.